Issue with Subdivide in 2.5
2.5 can't handle N-gons

Subdividing a single face implies that each edge of its boundary is getting new edges and vertices too. This means that the faces in the neighborhood are increasing their edge count.

If the subdivision process brings the faces to have a number of edges, like in your case, greater than 4, you are creating an ngon.

N-gons has been supported only since 2.63 (see Release notes).
 Before that version only quads and triangles were handled. That's why you see the subdivision.